One-port (two-terminal) devices exhibiting a negative differential resistance (NDR) in its current-voltage (I-V) relationship are locally-active circuit elements. With a proper external circuitry, so that the signal lies within the negative resistance region of the I-V curve, the device can have an AC power gain greater than 1 and serve as an amplifier, or excite oscillations in a resonant circuit to make an oscillator. Unlike in a two-port amplifying device such as a transistor or operational amplifier, the input signal and the amplified output signal share the same two terminals (port) of the device.
Generally NDR devices are classified into two categories: current-controlled (CC-NDR), or S-type (both terms, CC-NDR and S-type NDR, will be used equivalently in this presentation); and voltage-controlled (VC-NDR), or N-type. N-type NDR devices are readily available and come in a variety of device structures and operating mechanisms, including Esaki diodes, resonant tunnel diodes (RTD), Gunn diodes, impact ionization avalanche transit time (IMPATT) diodes, and tunnel injection transit time (TUNNETT) diodes. However, these devices are normally “On”, meaning that they are at low-resistance states when powered off, and therefore are not suitable for applications that require low standby power dissipation, such as spike-based neuromorphic computing.
On the other hand, S-type NDR devices are normally “Off”, meaning that they are at high-resistance states when powered off, and therefore are well suited for applications that require very low standby power dissipation. However, S-type NDR devices are rare and not readily available. A familiar type of S-type NDR device is threshold switch, such as Si PNPN devices (or thyristors), including unidirectional silicon-controlled rectifiers (SCRs) or programmable unijunction transistors (PUTs), and bidirectional triacs. The equivalent circuit of a thyristor is a pair of PNP and NPN bipolar junction transistors (BJTs) with appropriate connections to form an internal feedback loop. However, Si thyristors are mainly used for power control applications, the minimal threshold voltage (current) of commercially available discrete Si thyristors are typically 7-9V (200 μA) or larger, which is too high for many low-power applications. Although the threshold voltage can be reduced to 1-2V range by controlling the gate trigger current, additional circuit element such as a low-breakdown-voltage Zener diode is required, which adds the circuit overhead and complexity. Another important drawback of Si based PNPN devices is that they are non-stackable and have poor lateral scalability, thus severely limiting the network scale and device density for neuromorphic applications.
For spiking-neuron based neuromorphic computing applications, S-type NDR MOM threshold switching devices are envisioned to have advantages in scalability, switching speed, energy consumption, and biological fidelity, as compared with existing Si CMOS based solutions: superior scalability owing to the MOM crossbar geometry with a 4F2 scaling, F being the half pitch of lithography; superior stackability owing to the thin-film deposition fabrication process of the active layer, effectively enhance the 4F2 scaling to 4F2/N (N being the number of the active layers); less-than 10 picosecond switching speed owing to the ultra-fast Mott Insulator-to-Metal Transition (IMT); ultra-low energy consumption owing to the low IMT transition temperature and scalable active element; and higher biological fidelity and complexity owing to the inherent biomimetic nonlinear dynamics and stochasticity.
Certain types of MOM devices with a thin layer of transition metal oxides (TMO) are known to be threshold switches. Several TMO materials are known to follow Mott physics and possess a thermodynamically driven simultaneous structural and electronic first-order quantum phase transition from an insulator to metal as the material is heated beyond a characteristic critical temperature. Examples include binary oxides with Magneli phases, MnO2n-1 (M being V, Nb, Ti cations, n being an integer); or ternary perovskite-type oxides, RMO3 (R being rare earth cations such as Pr, Nd, Sm; M being 3d transition metals such as Ni and Co). However, many such materials have a cryogenic transition temperature, making it challenging for typical electronic applications. Materials with a transition temperature at above room temperature, such as VO2, NbO2, Ti2O3, and Ti3O3, are more suitable for such applications.
The reference: “Current-Induced Electrical Self-Oscillations Across Out-Of-Plane Threshold Switches Based on VO2 Layers Integrated In Crossbars Geometry” by A. Beaumont, J. Leroy, J.-C. Orlianges, and A. Crunteanu, J. Appl. Phys. 115, 154502 (2014), describes a VO2-based vertical MOM crossbar threshold switch with threshold voltages down to 0.8V. In this reference, the VO2 films were deposited on c-sapphire substrates at temperatures near 500 degree C. FIG. 1 schematically shows an elevation view of a MOM switch 10 of this reference, having a first metal electrode 12 formed on a c-sapphire substrate 14, both the electrode 12 and the substrate 14 not covered by the electrode being covered by a blanket film/layer 16 of VO2. A second metal electrode 18 is formed on top layer 16, thus sandwiching a portion of layer 16 between electrodes 12 and 18 at a crossing point 20. The Inventors have noted that a main issue of this known device is that the process using sapphire substrates and high growth temperatures is not compatible with Si CMOS back end of line (BEOL) processes, which posts a major barrier for adoption in large-scale integrated circuits (ICs) required for neuromorphic applications. Most of such ICs still require CMOS peripheral circuitry to support functions such as voltage supplies, data communication, analog/digital conversion, and input/output interfaces.
The reference “Filament Formation in Switching Devices Based on V2O5 Gel Films”, by J-G. Zhang and P. C. Eklund, J. Mater. Res. 8, 558 (1993), describes electroforming of lateral MOM devices (with a metal-to-metal gap of 150 μm) based on V2O5.1.6H2O sol gel films. S-type NDR I-V characteristics and resistance switching were observed after electroforming by applying a voltage typically 25-30V. The switching threshold voltage of the MOM devices of this reference is typically 10V. Further, the sol-gel film process and large metal-to-metal gap (150 μm) of the lateral device structure of this reference are not practical for IC applications.
The reference “A scalable neuristor built with Mott memristors” by M. D. Pickett, G. Medeiros-Ribeiro and R. S. Williams, Nature Mater. 12, 114 (2013), discloses using 6V and 1 microsecond electroforming pulses to form local NbO2 channel in a Nb2O5 crossbar device.
What is needed is an IC process compatible S-type NDR MOM low voltage threshold switching device using a TMO having a transition temperature at above room temperature. The embodiments of this presentation address these and other needs.